Non-woven webs are incorporated into a diversity of consumer and industrial products, including disposable hygienic articles, throwaway protective apparel, fluid filtration media, and household durables. Generally, non-woven webs are formed using melt-spinning technologies, such as spunbonding processes and meltblowing processes, that form continuous filaments or fibers composed of one or more thermoplastic polymers. Spunbond non-woven webs are relatively strong in both the machine and the cross-machine directions because of drawing that aligns the polymer molecules. The continuity of the filaments also contributes to the observed strength of spunbond non-woven webs. Spunbond non-woven webs also resist abrasion, have a high porosity, and may be soft and conformable.
Spunbonding processes generally involve pumping one or more molten thermoplastic polymers through a spin pack that distributes, filters, combines, and finally extrudes continuous filaments of the constituent thermoplastic polymer(s) through hundreds or thousands of spinneret holes or orifices in a spinneret. After extrusion, the filaments are cooled or quenched to increase their viscosity and then drawn or stretched by an impinging high-velocity airflow generally capable of orienting the molecules of each constituent thermoplastic polymer if the air velocity is sufficiently high. The airflow propels the drawn filaments toward a forming zone to form a non-woven web on a moving collector.
The spin pack distributes a flow of each constituent thermoplastic polymer from a few inlet ports to individual outlet ports that span the width of the spin pack. Specifically, the molten thermoplastic polymer from each inlet port is directed into a shared lateral flow passageway and individual portions of the incoming thermoplastic polymer are allocated from the lateral flow passageway to the outlet ports for subsequent distribution to the orifices in the spinneret plate. Because all of the inlet ports share a single lateral flow passageway, thermoplastic material streaming from adjacent inlet ports into the lateral flow passageway intersects, collides and mixes before arriving at the outlet ports. The intersecting streams of molten thermoplastic polymer may experience hold-ups, dead spots or stagnation zones, and/or recirculation within the lateral flow passageway. The individual streams of the polymer(s) from the outlet ports are ultimately supplied to the orifices in the spinneret.
The inability to uniformly divide the incoming stream of the molten thermoplastic polymer in the machine direction and in the cross-machine direction with uniform flow characteristics to the outlet ports causes unacceptable variations in the non-woven web formed by the spunbonding process. For example, non-uniform distribution of the molten thermoplastic polymer in cross-machine direction may cause the basis weight of the non-woven web to fluctuate in the cross-machine direction, which produces perceptible strips of varying basis weight extending parallel to the machine direction. In particular, the basis weight of the non-woven web originating from filaments extruded from spinneret orifices receiving thermoplastic polymer from outlet ports directly downstream of an inlet port has been observed to be significantly larger than the basis weight of the non-woven web originating from filaments extruded from spinneret orifices receiving thermoplastic polymer from outlet ports near the mid-point between adjacent inlet ports. The fluctuation in the basis weight is believed to arise from unequal flow path lengths in the shared lateral flow passageway. This results in non-uniform residence times and pressure drops for different portions of the non-Newtonian thermoplastic polymer exiting the outlet ports from the lateral flow passageway. The non-uniform flow path lengths also result in disparate shear histories for different portions of the thermoplastic polymer flowing in the lateral flow passageway reflected in the polymer properties and the characteristics of the non-woven web formed therefrom.
It would be desirable, therefore, to provide a spin pack for a melt-spinning apparatus capable of forming a non-woven web having improved basis weight uniformity in the cross-machine direction.